1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device implementing in-plane switching (IPS) where an electric field to be applied to liquid crystal is generated in a plane parallel to a substrate.
2. Discussion of the Related Art
A conventional liquid crystal display (LCD) device uses optical anisotropy and polarization properties of liquid crystal molecules. The liquid crystal molecules have a definite orientational order in alignment resulting from their thin and long shapes. The alignment direction of the liquid crystal molecules can be controlled by applying an electric field to the liquid crystal molecules. In other words, as the alignment direction of the electric field is changed, the alignment of the liquid crystal molecules also changes. Since the incident light is refracted to the orientation of the liquid crystal molecules due to the optical anisotropy of the aligned liquid crystal molecules, images are displayed.
Of the different types of known LCDs, active matrix LCDs (AM-LCDs), which have thin film transistors and pixel electrodes arranged in a matrix form, are the subject of significant research and development because of their high resolution and superiority in displaying moving images. Driving methods for such LCDs typically include a twisted nematic (TN) mode and a super twisted nematic (STN) mode.
FIG. 1 is a schematic perspective view of a conventional liquid crystal display device.
In FIG. 1, the conventional LCD device is composed of upper and lower substrates 5 and 22. A black matrix 6, a color filter 7, which includes sub-color filters (red, green, blue) 8, and a transparent common electrode 18, which is disposed above the color filter 7, are formed on the upper substrate 5. A pixel region “P”, a pixel electrode 17, which is disposed at the pixel region “P”, and an array line, which includes a switching device “T”, are formed on the lower substrate 22. A liquid crystal layer 14 is interposed between the upper and lower substrates 5 and 22. The black matrix 6 is formed by deposition and patterning of an opaque metallic material having a low reflectance or by coating and patterning an opaque photosensitive resin.
The lower substrate 22 is commonly referred to as an array substrate, where thin film transistors “T” are arranged in a matrix configuration and are located where gate and data lines 13 and 15 cross. The pixel region “P” is defined by the gate and data lines 13 and 15, and a transparent conductive metal like indium-tin-oxide (ITO), for example, whose transmittance is relatively high, is used as pixel electrode 17 on the pixel region “P”.
If a voltage is applied to the common electrode 18 of the upper-substrate 5 and the pixel electrode 17 of the lower substrate 22, transmittance of the LCD device is changed according to an alignment state of the liquid crystal layer 14 so that images can be displayed.
The conventional LCD device having the above-mentioned structure, in which the liquid crystal layer is driven by the electric field perpendicular to the upper and lower substrates, has a high transmittance and a high aperture ratio. The common electrode of the upper substrate is grounded so that damage to the device due to the static electricity is prevented. However, the viewing angle of the conventional LCD device having the above-mentioned structure is narrow. Therefore, to overcome the drawback, new technologies are suggested. An in-plane switching (IPS) LCD device is one of the most researched new technologies. A detailed explanation about operation modes of a conventional IPS-LCD device will be provided with reference to FIGS. 2 to 3D.
FIG. 2 is a schematic cross-sectional view of a conventional IPS-LCD device.
In FIG. 2, upper and lower substrates 5 and 22 are spaced apart from each other, and a liquid crystal layer 14 is interposed therebetween. Both pixel and common electrodes 17 and 18 are disposed on the lower substrate 22. A color filter 7 is disposed on a surface of the upper substrate 5 and opposes the lower substrate 22. The pixel and common electrodes 17 and 18 apply an electric field 35 to the liquid crystal, in which the electric field 35 is parallel to the upper and lower substrates 5 and 22.
FIGS. 3A to 3D are schematic views conceptually showing operation modes of a conventional IPS-LCD device.
FIGS. 3A and 3B show the off-state of the IPS-LCD device. In the off-state there is no electric field between the pixel and the common electrodes 17 and 18, and the phase transition of the liquid crystal layer 14 does not occur. For example, the liquid crystal layer 14 has an angle of 45 degrees between the long axis and the direction parallel to upper and lower substrates 5 and 22.
FIGS. 3C and 3D show the on-state, in which a voltage is applied to the pixel and common electrodes 17 and 18 so that an electric field 35 parallel to the upper and lower substrates 5 and 22 is generated, and the phase transition of the liquid crystal layer 14 occurs. The liquid crystal layer 14 is twisted so as to have an twist angle of 45 degrees with respect to the off-state of FIGS. 3A and 3B and aligned to the generated electric field 35.
By the above-mentioned operation modes and with additional parts such as polarizers and alignment layers, the IPS-LCD device displays images. The IPS-LCD device has a wide viewing angle and low color dispersion characteristic. Specifically, the viewing angle of the IPS-LCD device is about 85 degrees in direction of up, down, right, and left. In addition, the fabricating processes of this IPS-LCD device are simpler than other various LCD devices.
However, because the pixel and common electrodes 17 and 18 are disposed on the same plane on the lower substrate, the transmittance and aperture ratio are low. In addition, response time according to a driving voltage should be improved and a cell gap should be uniform because of the low alignment margin.
Therefore, the IPS-LCD device can be adopted considering the advantages and the disadvantages.
FIG. 4 is a schematic plan view of an array substrate for a conventional IPS-LCD device.
In FIG. 4, row gate and common lines 50 and 54 are disposed parallel to each other and a column data line 60 is disposed perpendicular to the row gate and common lines 50 and 54. Moreover, a gate electrode 52 is formed at a specific portion of the gate line 50. A source electrode 62 is formed at a portion adjacent to the gate electrode 52 on the data line 60 and overlapping the gate electrode 52. A drain electrode 64 is formed at a portion corresponding to the source electrode 62 with a specific distance between the source electrode 62 and the drain electrode 64. Furthermore, in a pixel region defined by the gate and data lines 50 and 60, a plurality of common electrodes 55 extending from the common line 54, a first pixel line 66 extending from the drain electrode 64, a plurality of pixel electrodes 67 extending from the first pixel line 66 and a second pixel line 68 connecting the plurality of pixel electrodes 67 are formed. The common and pixel electrodes 55 and 67 are disposed parallel and alternating to each other. A storage capacitor “C” is formed on a specific region of the common line 54 by using the common line 54 and the second pixel line 68 as first and second electrodes of the storage capacitor C.
In the conventional IPS-LCD device having the above-mentioned structure, since the common line adjacent to the gate line is formed of the same material and in the same layer as the gate line, the gate and common lines can be electrically connected by a fabrication error. To prevent this problem, the gate and common lines should have a specific distance. However, since the liquid crystal layer of the area “A” on the distance between the gate and common lines cannot be operated normally, the aperture ratio is remarkably decreased. Moreover, since the voltage difference between the gate and common lines always exists, the reliability of the IPS-LCD device is also decreased.